Improvement in strength of hardened cement is an ever-continuing subject of study. The ultimate aim of the study is reduction in production cost.
The subject has been approached from the angles of (1) material and (2) working method. The subject from the angle of material (1) is further divided into (a) improvement in chemical and physical characteristics of cement and (b) utility of additives, and much study has been given to a combination thereof as well. As examples of (a), improved cement obtained by finely grinding ordinary cement to a specific surface area by the Blaine method of 8,000 cm.sup.2 /g or more (as described in JP-A-2-208252, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), hydraulic cement mainly composed of three groups of particles each having a particle diameter of not more than 10 .mu.m, a particle diameter of from 20 to 45 .mu.m and a particle diameter of from 63 to 150 .mu.m (as described in JP-A-1-242445) and an analogous technique (as described in JP-B-2-31020 and U.S. Pat. No. 4,353,746, the term "JP-B" as used herein means an "examined published Japanese patent application") have been proposed. As examples of (b), a combination of a water reducing agent and silica fume (as described in JP-B-60-59182 and Canadian Patent 1,190,947) has been proposed.
JP-A-62-162506 and U.S. Pat. No. 4,915,741 propose a composition of water-added cement which is prepared by adding 0.1 to 10% by weight of water to ordinary cement and mixing the resulting water-added cement with a water reducing agent. According to the disclosure, the composition of water-added cement exhibits satisfactory fluidity, a satisfactory water reducing effect and improved strength.
However, even with the cement proposed to date and with careful working, the improvement attained in strength is not higher than about 10 to 15% over the strength obtained with commercially available cement, and no further improvement can be expected.